1. Field of the Invention
The present invention describes compositions useful in preparing coatings for substrates such as metals, plastic, glass, wood and other such materials.
2. Description of the Art
Epoxy resins having more than one epoxide group per mole of the resin are known in the art to form hard insoluble and infusible products. It is also known that compounds having primary or secondary amine structures will react with epoxy resins. Polyamines, that is, compounds having more than one amine group are utilized to cure such epoxy resins.
U.S. Pat. No. 3,280,074 issued Oct. 18, 1966 to McCaleb et al. states that complex amines having a monotertiarydiprimary structure may be used as curing agents for the epoxy resins described above.
While the amines described in the McCaleb et al. patent function well for their intended purpose it cannot be said that these materials could not be improved upon. Among the areas of improvement that are encompassed by the present invention include the observation that the processing of the materials in the McCaleb patent require large amounts of hydrogenation catalysts which have been found to be deactivated in short order by the acetic or phosphoric acid used in preparing the final amine product therein. The acid is needed to ensure that the secondary amine will react with the nitrile compound.
To counteract the effect of the acid deactivation of the hydrogenation catalyst it has been proposed to neutralize the acid with a caustic material. This, however, results in salt formation and a second step must be employed to remove the salt by washing with copious amounts of water. It is also observed that the water deactivates the hydrogenation catalyst when present at any substantial level during the hydrogenation reaction. Thus, while the acid may be neutralized and the resulting salts washed out of the reaction mixture it is necessary to employ yet a further step, usually evaporation, to remove the water from the reaction mixture to avoid a second source of catalyst deactivation.
It has been found in the present invention that the monotertiarymonosecondarydiprimarytetramines (also referred to as tetram) and the precursor monotertiarymonosecondarydinitrilodiamines require no acid utilization in their preparation and thus the neutralization, water washing and subsequent water removal are avoided in forming the claimed compounds of the present invention.
A second advantage in avoiding the acid used in the prior art is that a single solvent may be employed, usually methanol, throughout the reaction process to form the tetram. Therefore while the process of McCaleb et al also uses methanol that solvent must be stripped out during the water washing and recovered for subsequent use whereas in the present invention the methanol need not be reclaimed until the final step of tetram formation. It must be remembered that in McCaleb et al. that the solvent would also be reclaimed following the final step thus the present invention requires only one purification step for reuse of the solvent. Therefore the amount of valuable product which is carried off and lost with the solvent can be twice as great in McCaleb et al than in the present invention. Further, the solvent which can be lost in the atmosphere due to two recovery steps is greater than the one step solvent recovery in the present invention.
It has also been observed that the tetrams of the present invention have a longer pot life and form harder coatings upon curing the epoxy resins than do the compositions of McCaleb et al. While the secondary amine structure in McCaleb et al. causes processing difficulties and is eventually obliterated to form the end polyamine compound therein the present invention retains a secondary amine group which causes greater crosslinking with the epoxy resin. Pot life refers to the duration of time between the mixing of the tetram and the epoxy resin and the point at which the mixture becomes too viscous to apply to the substrate. It is universally desired that the pot life of any given coating agent be sufficiently long to avoid hardening of the mixture in the mixing vessel or pot. While the compounds of the present invention do exhibit long pot life they are particularly advantageous in that they cure quickly upon application to the substrate. Moreover, the coating formed by the tetram of the present invention in combination with the epoxy resin does not carbonate, i.e. turn white upon application to the substrate. This is particularily valuable when working with unpigmented coatings requiring a clear finish on the substrate. In addition, the non-carbonating feature of the tetram and epoxy resin cured mixture provides greater strength than prior art epoxy resin compositions which do undergo carbonation in curing.
It has also been observed that the tetrams of the present invention are liquids at room temperature and thus do not require the expenditure of costly energy to melt the tetram prior to combination with the epoxy resin. Moreover, the tetrams of the present invention are not only liquid but are of sufficiently low viscosity that mixing is easily accomplished upon combination with the epoxy resin in the pot.
It has also been found that the precursor of the tetram namely the monotertiarymonosecondarydinitrilodiamine is itself a preferred curing agent for several resin compositions. Such resin compositions include the reaction product of the dinitrilodiamine with olefins (alkenyl) to give substituted fatty acid amides in the presence of sulfuric acid catalyst. The dinitrilodiamines of the present invention may also be reacted with water in the presence of acid catalysts to give the corresponding fatty diamides and with hydrogen sulfide to give fatty dithioldiamide compounds. Thus the dinitrilodiamines are useful for the preparation of a host of compounds including the tetrams of the present invention.
Thoughout the specification and claims percentages and ratios are by weight and temperatures are in degrees Celsius unless otherwise indicated.